A radio channel experiences severe channel changes according to the time, compared to the wired channel. To achieve a required performance, an optimum transmission scheme (a modulation scheme, a code rate, a transmit power, and so forth) is determined in every transmission time. In order to accomplish this, a base station needs to acquire accurate channel information of each individual terminal. In an Institute of Electrical and Electronics Engineers (IEEE) 802.16 system, the terminal periodically reports its measured channel information of the downlink to the base station over a CQI channel.
Meanwhile, since the feedback of the CQI information requires at least one frame in a Time Division Multiplexing (TDM) system, the time point of the channel quality measurement by the terminal is different from the time point of the actual data transmission in the downlink. Accordingly, in a wireless channel environment under the severe channel variation based on the time, the CQI information includes some error and thus the performance degradation is caused. Namely, when the channel quality further degrades in the channel measurement of the terminal and in the actual signal reception, the transmission error is caused. In addition, the base station may not receive the CQI information because of the influence of the serious fading or the strong interference signal and may degrade the system performance. In the former case, to mitigate the performance degradation caused by the channel variation according to the time, a retransmission scheme such as Hybrid-Automatic Repeat Request (H-ARQ) is adopted. In the latter case, the error of the information bits can be minimized by applying a more robust code rate to the control information such as CQI information. The IEEE 802.16 system supports two CQI channel operation manners according to a subchannel allocation scheme; that is, supports a case where a diversity subchannel is allocated and a case where a band Adaptive Modulation and Coding (AMC) subchannel is allocated. The meaning of the CQI channel differs in each case. That is, when the CQI information relating to a plurality of bands is transmitted, much information is required to transmit the unique CQI value of each band.
In the IEEE 802. 16 system, the terminal sends a REP-RSP message to the base station for the allocation of the band AMC subchannel. When the base station does not correctly receive the REP-RSP message, it regards a Carrier to Interference and Noise Ratio (CINR) value (or the CQI information) relating to a certain band sent from the terminal as an average CINR of the entire channel. As a result, the base station may not reflect the accurate CINR value and the performance degradation are caused.
The REP-RSP message includes a Media Access Control (MAC) header and a Cyclic Redundancy Check (CRC) in the form of a MAC message, which leads to the considerable overhead. To reduce the overhead, a feedback header type may be utilized. However, this may degrade the performance as well when the base station does not correctly receive the feedback header type.
The CQI information transmission of the selected band causes the overhead in that the information of the selected band needs to be transmitted every time its modification is required. When the base station merely schedules with the selected band, the optimum scheduling for the unreported band is impossible because the entire band may not be filled only with the bands selected by the terminal.
Therefore, what is needed are methods and apparatuses for minimizing the overhead in the CQI transmission and for minimizing the error in the broadband wireless communication system.